1. Field of the Invention
The present invention relates to a process and a composition for forming flexible molds for finely detailed objects, to the molds produced thereby, and to a process for using the molds produced to cast such objects.
2. Brief Description of the Prior Art
Flexible molds for finely detailed objects are produced by two principal prior art processes.
A method widely used in the jewelry industry requires a three-dimensional model made of metal. The model is placed between sheets of a solid but unvulcanized rubber composition and heat and pressure are applied to embed the model in the rubber sheets and to vulcanize the rubber. A skilled craftsman carefully cuts apart the resulting block of opaque rubber surrounding the model to produce a two-piece flexible mold. This mold is then held together between metal plates, and wax is injected into a sprue opening to make duplicate waxes for the "lost wax" or investment casting of jewelry.
Other industries working with small models often use polymeric silicone compositions for casting flexible molds. Silicone molding compositions are usually two-part materials which must be mixed immediately before the mold is to be made. Both RTV ("room temperature vulcanizing") and elevated temperature cure formulations are available. Another type of two-part molding composition sometimes used is based on urethane cure chemistry. The use of two-part compositions requires measuring out catalyst and an uncured polymer composition in precise proportions and careful but thorough mixing. A vacuum source is often used to remove air bubbles entrained by the mixing. A two-piece mold can be made sequentially. However, the first piece of the mold must be completely cured before the second piece of the mold can be cast. Thus, from about 24 to 72 hours may be required to finish a mold. Alternatively, an elevated temperature cure material can be employed to encapsulate the model and the cured mold has to be carefully cut apart.
The use of radiant energy to cure compositions including ethylenically unsaturated components is well known. Typically, the radiation curable composition includes a component which is activated by the radiation and provides a free radical species. The free radical species initiates the reaction of the ethylenically unsaturated components of the composition. Often the cure entails the free radical-induced polymerization of ethylenically unsaturated monomers. Commonly, a high energy source of radiant energy is used, such as an ultraviolet (uv) lamp.
Most radiant energy-curable compositions are used to provide relatively thin coatings or layers of cured material. Common examples are uv-curable photoresists (such as disclosed, for example, in U.S. Pat. No. 3,661,576), printing inks (such as disclosed, for example, in U.S. Pat. Nos. 3,551,235 and 3,551,311), protective coatings for metal sheets and foils, glass, shaped polymeric solids (such as disclosed, for example in U.S. Pat. No. 3,719,522), wood veneer, plywood, chipboard, paper, cardboard, and the like, optical coatings for photographic film, and high gloss finish coatings. These coatings frequently include components which absorb or scatter uv radiation, and thus must be applied in a thin enough layer so that the entire thickness is effectively cured, or a second, usually thermal, step must be undertaken to complete the cure of the coating, as discussed in U.S. Pat. No. 4,073,835.
Radiative cure of a fluid composition to form three-dimensional objects is disclosed in U.S. Pat. No. 4,752,498, which relates to the use of a patterned "negative" to modulate the transmission of uv radiation. The more opaque portions of negative transmit less uv radiation, and the curable composition is disclosed to be cured to a lesser depth, presumably because the uv radiation effectively penetrates to a lesser depth. This "selective solidification" provides three-dimensional objects useful as printed circuit boards, and which can be metallized in a subsequent step. It is unclear how a clear demarcation between the surface of three-dimensional object and the sol fraction formed immediately adjacent thereto is provided. U.S. Pat. No. 2,525,664 similarly discloses varying the flux of radiation impinging on a fluid casting composition using a screen, but the goal here is to eliminate small voids and internal strains occurring when optical component castings are uniformly irradiated.
U.S. Pat. No. 4,042,654 relates to the radiative cure of thin plastic parts which are cast from a resin composition having sufficient viscosity such that the shape of the casting is retained, even in the absence of a mold, long enough for the casting to be radiatively cured.
A great variety of radiatively curable materials are known, including monomers and low-to-moderate molecular weight polymers (oligomers). Often a radiatively curable composition for surface coatings will include an oligomer component having one or more reactive sites and a reactive diluent component comprising one or more monomeric species. For example, U.S. Pat. No. 4,421,782 discloses a coating composition for floor tiles comprising about one-third reactive diluent and two-thirds acrylate-capped urethane prepolymer (Example I). Surface coatings prepared using such compositions are typically highly crosslinked. These high modulus, tough coatings simulate the performance previously obtainable only by environmentally unsound solvent-based coating compositions and coil coatings cured at at elevated temperature.
While there are a number of known materials and processes for preparing flexible molds for casting finely detailed three-dimensional objects, such as jewelry, objects d'art, and the like, there remains a need for a process which quickly provides a flexible mold for such objects. Further, there is a need for a process for preparing flexible molds which can be used immediately after preparation for molding reproductions of the model used. Similarly, there is a need for a molding process which can be readily employed by semi-skilled professional and amateur workers to quickly and accurately reproduce finely detailed three-dimensional objects.